Add link to geodesic modes

[zakjan]

I have created geodesic modes plugin. Most of the modes simply inherit from the original MapboxDraw modes, and wrap toDisplayFeatures function with geodesic calculation.

const DrawLineString = MapboxDraw.modes[Constants.modes.DRAW_LINE_STRING];

const DrawLineStringGeodesic = { ...DrawLineString };

DrawLineStringGeodesic.toDisplayFeatures = function(state, geojson, display) {
  const displayGeodesic = (geojson) => {
    const geodesicGeojson = createGeodesicFeature(geojson, { ctx: this._ctx });
    geodesicGeojson.forEach(display);
  };

  DrawLineString.toDisplayFeatures.call(this, state, geojson, displayGeodesic);
};

There is also a static mode which can be used to render geodesic map features without using the drawing capabilities. It is useful because geodesic calculations are isolated inside the plugin, keeping user code abstracted away from the calculations.

Related to mapbox/mapbox-gl-js#3544